Aqueous solubility of calcium citrate and interconversion between the tetrahydrate and the hexahydrate as a balance between endothermic dissolution and exothermic complex formation

Aqueous solubility of calcium citrate tetrahydrate was found to decrease with increasing temperature, while solubility of hexahydrate increased with a transition temperature at 51.6 °C. Excess citrate increased calcium citrate solubility but decreased the calcium ion activity of the saturated solution with an initial solubility overshooting to form supersaturated solutions indicating binding of calcium to citrate with an association constant of 3.6 ± 0.1 × 104, ΔHº = −5.07 ± 0.04 kJ mol−1, ΔSº = 70.3 ± 0.3 J mol−1 K−1 at 25 °C. Dissolution of the tetrahydrate and hexahydrate was found to have ΔHº = 27 ± 9 kJ mol−1, ΔSº = −218 ± 30 J mol−1 K−1 and ΔHº = 57 ± 7 kJ mol−1, ΔSº = −126 ± 24 J mol−1 K−1, respectively, as determined from the temperature dependence of solubility corrected for complex formation. The exothermic complex formation results in inverse solubility only for the tetrahydrate with its moderate endothermic dissolution, which also precipitates at ambient temperature rather than the less soluble hexahydrate.